1. Field of the Invention
This invention relates to a motor controller, and more particularly, to a motor controller for controlling a multi-phase bipolar parallel winding brushless DC motor.
2. Description of Related Art
In a multi-phase bipolar brushless DC motor, a permanent magnet having a plurality of poles is used instead of field coils for the rotor and an armature is used for the stator. There is a plurality of coils wound on the stator, each coil representing an independent phase winding. For example, a stator with 6 phases has 6 independent windings, each winding presenting one phase. This type of motor is described in U.S. Pat. No. 4,882,524 issued Nov. 21, 1989 to Lee, which is incorporated by reference as if stated fully herein.
The typical multi-phase DC motor has a stator with a number of phases. Each phase has a plurality of windings, which are connected in series and positioned around the circumference of the rotor. In addition, each phase is connected independent of the winding connection of the other phases. The rotor comprises a number of permanent magnetic poles and rotates in response to the direction of the current in the windings.
To drive the multi-phase DC motor, a commutation encoder and photo interrupters or hall effect devices work in combination to provide synchronized currents to the motor. For example, a photo interrupter, which is responsive to a light source, is turned on or off depending on the position of a commutation encoder and controls a transistor connected to one of the stator windings. A pair of photo interrupters per each phase is arranged in the commutation encoder. Such an arrangement allows each phase to be enabled in a sequential manner so that the maximum current can be applied.
Alternative to using the photo interrupter and commutation encoder is to use a pulse width modulation (PWM) technique to control speed of the motor. The PWM technique simply generates a pulse having a predetermined duty cycle to drive all of the phase windings. As the pulse has a longer on period, the faster the motor rotates.
However, the disadvantage of the above configuration is that the current applied to each phase winding controls the torque and speed of the motor. Because all of the phases must be turned on in a sequential or a predetermined order to maintain the rotation of the motor, current is provided to all of the phase windings.
A conventional DC motor control system or brushless DC motor control system utilizes a PWM method with one or one-half H-bridge configuration. As a result, the electrical current to the motor must flow through the only H-bridge coupled to the DC motor. The H-bridge, therefore, is typically made with power MOSFET's or Isolated Gate Bipolar Transistors (IGBT). For example, a MOSFET of a conventional H-bridge must handle 100 amps of current. In addition, because of the high current requirement of the PWM, there are many limitations to the controller circuit caused by excessive heat dissipation from the internal circuit semiconductor devices, which are sensitive to heat, and high current.